Micro-electromechanical device and module and method of manufacturing same

ABSTRACT

The MEMS element of the invention has a first, a second and an intermediate third electrode. It is given an increased dynamic range in that the switchable capacitor constituted by the second and the third electrode is provided in the signal path between input and output, and that the switchable capacitor constituted by the first and third electrode is provided between the signal path and ground. The MEMS element of the invention is very suitable for integration in a network of passive components.

The invention relates to an electronic device comprising amicroelectromechanical system (MEMS) element, the element having a firstand a second electrode and an intermediate beam with a first and asecond opposite conductive side faces, the first side face facing thefirst electrode and the second side face facing the second electrode,which beam is movable to and from the first and the second electrode byapplication of a driving voltage.

The invention further relates to a module comprising such a device.

Such an electronic device is for instance known from WO-A 00/52722. Theknown device is a MEMS element in which the electrodes and the beam areprovided in planes substantially parallel to a substrate. Theintermediate beam is herein a laminate comprising between the twoconductive surfaces a first insulating layer, a cantilever beam and asecond insulating layer. The side faces are used as control surfaces andprovide the capability to use separate driving functions for the top andthe bottom switch structures. This allows for simultaneous push and pulloperation for enhanced speed. By placing both conductive surfaces at theground common potential, an electrostatic shield between the signalcurrents at the cantilever beam and the control signals providing thecoulomb forces are provided. This structure thus provides signalisolation enhancement when compared to simpler cantilever beamstructures.

It is a disadvantage of the known electronic device that it has aninsufficient dynamic range for applications in the RF domain.

It is therefore a first object of the invention to provide an electronicdevice of the kind mentioned in the opening paragraph with an improveddynamic range.

In accordance with a first aspect of the present invention, there isprovided an electronic device comprising a micro-electromechanicalelement (MEMS) device, comprising first and second electrodes and anintermediate beam with first and second opposing conductive side faces,the first side face facing the first electrode and the second side facefacing the second electrode, which beam is movable by application of adriving voltage between said first and second electrodes, the devicebeing characterized in that the second electrode and the secondconductive side face of the beam form with an intermediate dielectric afirst switchable capacitor that is connected in a signal path between aninput and an output, and the first electrode and the first conductiveside face of the beam form with an intermediate dielectric a secondswitchable capacitor that is coupled from the signal path to ground.

Surprisingly it has been found that the MEMS element with thisconnection, wherein one capacitor is in the signal path and the other isconnected to ground, provides an improved dynamic range. This connectionis in fact a combination of the shunt and the series configuration ofconventional MEMS capacitors and switch without any intermediate beam.The element of the invention leads in comparison therewith to asignificant higher isolation for a given insertion loss. Whereasconventional RF MEMS capacitive switch show an isolation of −20 dB for−0.1 dB of insertion loss, the element of the invention shows anisolation of −32 dB at the same insertion loss. This improvedperformance is moreover achieved without increasing the overall devicedimensions.

In a first embodiment the beam is embodied as a third electrode. Thus, amuch simpler construction for the beam is used than in the state of theart. This has as a first advantage that it has a low stiffness and thusallows for very low actuation voltages, preferably below the batteryvoltages of a few volts. It has as a second advantage that themanufacturability is improved. In fact, the element of this embodimentcan be realized in a thin film process.

In a further embodiment, the surface area of the second electrode issmaller than that of the first electrode. Herewith the switchperformance can be tuned and an even higher isolation can be achieved.There are various ways of modifying the surface area of the secondelectrode; it could be smaller than the third electrode and the firstelectrode; it could have a limited overlap with the third electrodeonly. It is however preferable that the second electrode is subdividedinto individual segments. Such segmentation allows that the thirdelectrode falls within the boundaries of the second electrode. Thisallows for a good design of the surface area ratio between the first andthe second electrode. Furthermore, it creates space under the thirdelectrode. This has the benefit that the sticking problem of the thirdelectrode to the second electrode is diminished. If desired it could bediminished actively, using flow of air, or by filling the spaces betweenthe segments with another material that does not show any attractinginteraction with the material of the third electrode. With such anembodiment, the isolation can be improved to −38 dB. The effect ofsegmentation is considerable in total; for a fourfold increase of thecapacitor coupled to ground (with respect to the series capacitor), thedynamic range is enlarged tenfold.

It is preferred for this construction that the MEMS element is of thehorizontal type, e.g. that the first and second electrode are present inplanes that are substantially parallel oriented with respect to thesubstrate. The term substantially parallel should be seen in comparisonto the vertical type of MEMS elements, which is for instance used inaccelerometers.

It is further preferred that the second electrode is present on thesubstrate surface as it is easier to subdivide the second electrode intoindividual segments if it is present on the substrate.

It is even more preferred that the first electrode is embodied in alayer with a spring constant that is substantially larger than thespring constant of the beam. The first electrode is generallyconstructed in a bridge like form. For a stable operation, the movementof the bridge induced by the driving voltage should be negligiblecompared to the beam movement. Furthermore, there is a risk ofresonating behavior of this first electrode if the beam is moved upwardsor downwards if the bridge is not stiff enough. This is undesired.Increasing the stiffness of the layer of the first electrode takes awaythis problem. The stiffness of the layer can be increased by increasingthe thickness of the layer, for instance to the order of 1-10 microns,or by using a material with a higher stiffness that nevertheless hasgood electrical conductivity. Examples hereof are alloys of Al and Tiand of Al and Cu, with preferably 1-5% of the alloying element.

It is further preferred that the first electrode is connected to ground,since it has an increased thickness. The increased thickness results ina higher electrical conductivity, such that the ground has also a realground potential.

The term “conductive side faces” should not exclude the case that thesefaces are covered with a thin layer of insulating material. In fact, ifAl is used for the third electrode, a native oxide of Al₂O₃ can beformed. This is particularly preferred in the case where the first andsecond electrode not only function as signal electrodes, but also asactuation electrodes. Also the third electrode can be given bothfunctions. The dielectric serves then as a protection to preventshort-circuitry.

The first and second switchable capacitors can be applied both ascapacitors and as switches. If the capacitive behavior towards ground isdesired, a dielectric layer can be present on top of the secondelectrode. A suitable dielectric is for instance silicon nitride,tantalum oxide etc.

The invention further relates to an electronic device comprising amicroelectromechanical systems (MEMS) element provided on a substrate,comprising first and second electrodes, which electrodes are provided inplanes that are substantially parallel to the substrate, an intermediatebeam being provided between said first and second electrodes, saidintermediate beam having first and second opposing conductive sidefaces, the first side face facing the first electrode and the secondside face facing the second electrode, which beam is movable byapplication of a driving voltage between said first and secondelectrodes.

It is a second object of the invention to provide such an electronicdevice, that is easy to manufacture and enables superior electricalperformance.

This object is achieved in that the first and second conductive sidefaces are part of the same electrically conductive layer being a thirdelectrode. Herewith, use can be made of a thin-film process with threemetal layers. Such a thin-film process can be controlled more easily.

It is an advantage of the present invention, that this MEMS element withthree electrodes has good electrical performance, particularly ifconnected in the further shown manner. The third electrode can be putboth in contact with the first electrode and with the second electrode.If attached to the first electrode, the electrical contact is rathergood, resulting in a very low isolation loss. If attached to the secondelectrode, the isolation loss is rather high, up to −40 dB.

The properties of the element can be optimized for use as a capacitor,sensor or switch. If meant for use as a variable capacitor, the shapesof the first and second electrodes can be chosen differently, so as tohave another capacitive area between the first and the third electrodethan between the second and the third electrode.

If meant for use as a switch, the overlap between the electrodes can bereduced so as to prevent sticking behavior. Also, the area of contact atthe first and second electrodes need not be chosen at the same location;i.e. when projecting the first electrode on top of the metal layer ofthe second electrode, there need not be any overlap between the firstand the second electrode. This can be achieved in adequate patterning ofthe electrodes. This reduced area allows a larger freedom of design ofthe electrodes, and thus enables optimal RF properties of the device.The parasitic capacitance of the substrate may be reduced, and thelocation can be chosen such that interconnects may have striplinecharacter through coupling to a ground plane or ground interconnect(e.g. to act as a transmission line or coaxial structure).

An alternative implementation of the desired patterning of theelectrodes is that surface layers are provided at one or bothelectrodes, which include windows for making contact. The use of one ortwo of such surface layers may reduce any sticking behavior, as theinterface between the electrodes will not be completely flat. This willnot be detrimental to the electrical contact, as the results withoutsuch surface layer show that there is some space for compromising. Theuse of a surface layer is particularly suitable for the secondelectrode. If the MEMS element is integrated in a passive network withcapacitors, a patterned dielectric layer of relatively small thicknessis present anyway on top of the metal layer in which the secondelectrode is defined. There is no problem to pattern it adequately suchthat it the exposed area of the second electrode is smaller than that ofthe third electrode.

In a further embodiment, the MEMS element is provided with a spring likeelement in its construction. Such springs like constructions are knownper se in the field of MEMS elements. Basically, the bridge-like ormembrane-like construction, acting in the invention as the firstelectrode, is connected to the substrate through a number of beams thatare present laterally to this construction. These beams have a largerelasticity and hence can vibrate. Therewith they are equivalent tosprings. The beams may have a desired design which for instance includesan angle.

Such a construction is particularly advantageous in connection with theinvention in that it allows movement of the first electrode. Thisfurther extends the range of capacitances. Furthermore it allowsswitching between three or more states: the fully-closed state with allthree electrodes attached to each other; a first half-open state, withthe third electrode attached to the first electrode, a second half-openstate with the third electrode attached to the second electrode; andalso the fully-open state with the third electrode not being attached toeither the first or the second electrode. For this last state it ispreferred that also the third electrode is supported with a constructionincluding springs. Another advantage hereof is the fact that at the samedriving voltage a force which is for instance 6.75 times as high isavailable to counter any stiction force. This improves the reliabilityof the device.

In a suitable embodiment, the second electrode is substantially elastic.This allows that the second electrode can be rolled off. This means thatpart of the second electrode will be in a position attached to the firstelectrode, and part of the second electrode will be in a position verynear to the second electrode. The elastic behavior of the secondelectrode can be realized in the choice of the material and thethickness of the second electrode. Thin metal layers of for instancegold, silver, copper and aluminum are elastic enough. The elasticity canfurther be tuned by alloying it with suitable elements, for instance inthe case of aluminum with about 0.5-2% copper. This alloy shows asimilar hardness as pure aluminum, but has a substantially reducedcreep.

This embodiment is particularly suitable if the third electrode formswith at least one of the other electrodes forms a capacitor. Theadvantage is that the capacitor has a continuous tuning range that isconsiderably extended, particularly in comparison with a variablecapacitor having only a first and a second electrode.

The tuning of the capacitance is accomplished by changing the electrodearea and/or by using different dielectrics on the surfaces of the firstelectrode and of the second electrodes.

The first implementation is thus that the dielectric constant of thedielectric layers on the surfaces of the first and the second electrodeis different. It is not impossible to use on the one surface a layerwith a relatively high dielectric constant, such as silicon nitride,tantalum oxide or even perowskite ceramics such as lead zirconiumtitanate and the like, while on the other surface a layer with a lowdielectric constant is used, such as benzocyclobutene, a mesoporousorganically modified silica or the like.

The second implementation is that the first and second electrodes havedissimilar shapes. Such shapes may be that the electrodes are patternedto be present only locally. In view of the usual bridge-construction ofthe first electrode, this relates primarily to the shape of the second,bottom electrode. It may further be subdivided into a number ofsegments. Particularly suitable appears a segmentation into triangularsegments. This allows a continuous change of the electrode area and atthe same time results in the fact that the segments are mutuallyconnected inside this electrode.

It is highly preferred that the layer in which the first electrode ispresent has a sufficient mechanical strength so as to have a bridge-likeconstruction, and at the same time has a sufficient electricalconductivity for acting as interconnect in RF applications. This can beaccomplished by choosing the material adequately.

In a further embodiment, this MEMS element is integrated in a passivenetwork. Inductors can be defined in the layer in which also the firstelectrode is defined, and which is given a larger thickness and/oranother material composition, so as to increase its stiffness incomparison with the third electrode. Electrodes of thin film capacitorscan be defined in the same layers as the second and third electrodes.The dielectric layer thereof could even cover the second electrode ofthe MEMS element, if it is to be used specifically as a tunablecapacitor instead of a switch. The process of manufacturing such apassive integration process with good capacitors, good inductors andgood interconnects that is applicable in the RF domain, is known fromU.S. Pat. No. 6,538,874, that is included herein by reference.

The implementations and embodiments as described above are alsoapplicable here.

The substrate of this MEMS process is preferably insulating orsemi-insulating. Examples of such substrates include GaAs, glass,alumina, and ceramics with or without internal conductors. The choice ofthe ceramic may optimize the thermal expansion behavior. It is, however,preferred to use high-ohmic silicon as the substrate. Bothpolycrystalline silicon and high-ohmic monocrystalline silicon, that ismade high ohmic by implantation with ions such as He or Ar, can be used.A silicon substrate with an amorphous top layer is another suitableoption.

The electronic devices of the invention are very suitable for use inimpedance matching. Particularly preferred is the application ofimpedance matching networks for the antenna in mobile phones, whereinthe antenna switches for switching between the receive and the transmitpaths and between the different frequency bands are included. However,the impedance matching is also suitable at other locations, for instanceat the power amplifier, at the transceiver.

The available prior art does not teach the present invention.

Both EP1093142 and WO00/52722 show structures in which the intermediateelectrode is a laminate of several layers. That does not enable simplemanufacturing on an industrial scale.

US2002/0153236 shows a structure with an intermediate electrode (FIG.20), but this is a vertical MEMS construction (as used for sensors), nota horizontal structure.

U.S. Pat. No. 6,310,526 describes a structure with a magnetic actuationprinciple. Herein the MEMS is a switch between an input and two outputmicrostrips. Thickness values are given (0.5-10 um) for the microstrips,but this is dependent on the microstrip behavior solely. Neither is thesuggestion given to connect one of the two outputs to ground, and usethe switches as capacitors as well, nor is shown the manufacture of thedevice shown.

US2003/0048036 shows a MEMS structure that operates on the basis ofcomb-finger sensor-actuator. The patent discusses at the beginning theelectrostatic MEMS structure used in the invention, and states clearlythe differences to the comb-finger sensor/actuator.

These and other aspects of the present invention will be apparent fromand elucidated with reference to the embodiments described hereinafter.

Embodiments of the present invention will now be described by way ofexamples only and with reference to the accompanying drawings, in which:

FIG. 1A is a schematic cross-sectional view of a switchable MEMcapacitor;

FIG. 1B shows schematic circuit diagrams illustrating that the device ofFIG. 1A can either be used in shunt or series configuration;

FIG. 2 is a schematic cross-sectional view of a bi-stable MEMS elementaccording to an exemplary embodiment of the present invention;

FIG. 3 shows schematic circuit diagrams illustrating the operatingconfigurations of the device of FIG. 2;

FIG. 4 illustrates graphically that the device of FIG. 2 leads to asignificantly higher isolation for a given insertion loss relative toprior art devices;

FIG. 5 is a schematic cross-sectional view of a MEMS element accordingto another exemplary embodiment of the present invention;

FIG. 6 illustrates schematically the manufacturing flow for realizingthe device of FIG. 2;

FIG. 7 shows diagrammatically a cross-sectional view of a secondembodiment of the device of the invention;

FIG. 8 shows diagrammatically a cross-sectional view of a thirdembodiment of the device of the invention, and

FIG. 9 shows diagrammatically a top view of the third embodiment shownin FIG. 8.

Equal constituent parts in different figures will be referred to by thesame reference numbers. The drawings are purely diagrmmatical.

Referring to FIG. 1A of the drawings, a conventional switchable MEMcapacitor 10 comprises a body 12 mounted on a substrate 14 with a recess16 therebetween. A first electrode 30 is defined in the body 12. Asecond, fixed electrode 20 is provided on top of the substrate 14. Inuse, the free-standing first electrode 30 is pulled down to the bottomelectrode 20 by applying a DC drive voltage between the first and secondelectrodes 30, 20. In its extreme situation, the surface 18 of the firstelectrode 30 will contact the second electrode 20. As illustrated inFIG. 1B of the drawings, the MEM device 10 can be operated in either ashunt or series configuration.

In principle, capacitive MEMS switches, such as the one described above,offer a high isolation combined with a low insertion loss compared withtheir semiconductor counterparts (e.g. p-I-n diodes and field effecttransistors). As noted above, these conventional MEMS (capacitive, asopposed to galvanic contact) switches are used in either a shunt orseries configuration. In practice, however, the dynamic range of thistype of capacitive switch is limited by the capacitance density that canbe obtained when the top electrode is pulled down to the bottomelectrode 20. In particular, the surface roughness of the contactingelectrodes leads to a relatively low capacitance density when the switchis closed. For example, a 500 nm thick layer of sputtered aluminum has asurface roughness R_(d)˜nm. This leads to a residual air gap between theelectrodes of approximately 30 nm when they are pulled together. Inpractice, the effective air gap leads to a capacitance density of 300pF/mm², when the free-standing electrode is pulled down to the bottomelectrode 20. For a typical device layout, this leads to an isolation ofonly −20 dB for an insertion loss of −0.1 dB. The dynamic range can beincreased by increasing the dimensions of the switch 10 (i.e. byincreasing the electrode area and gap between the top and bottomelectrodes). In practice, however, this leads to unacceptably largedevices (i.e. electrode areas of several mm² and gaps>10 μm).

Referring now to FIG. 2 of the drawings, a MEM switchable capacitor 10according to an exemplary embodiment of the present invention comprisesa body 12 mounted on a substrate 14 with a recess 16 therebetween. Thefirst electrode 30 is defined in the body 12, and the second electrode20 is present on the substrate 14. The device further comprises anintermediate beam 220, comprising a free-standing thin film, between thefirst and second electrodes and substantially parallel thereto, theintermediate beam 220 forming a third electrode of the element. In otherwords, the device comprises three electrodes 30,20, 220, two of whichare suspended above the substrate 14. In use, the switching action isperformed by pulling the middle electrode 220 to either the top orbottom fixed electrode 30,20. This pulling action is established byapplying a DC voltage between the moving electrode 220 and one of thefixed electrodes 30,20.

In the arrangement shown in FIG. 2, each of the electrodes 30,20, 220 iscovered with a dielectric layer 240 so as to avoid a short circuit whenthe intermediate beam 220 is pulled to one of the fixed electrodes30,20. The dielectric layer present on the third electrode 220 is anative oxide of Al₂O₃ in particular, or any other insulating surfacelayer. The first electrode 30 and the first conductive face 260 of thebeam 220 form, with an intermediate dielectric, a first switchablecapacitor C1 that is connected in a signal path between an input and anoutput. Similarly, the second electrode 20 and the second conductiveside face 280 of the beam 220 form, with an intermediate dielectric, asecond switchable capacitor C2 that is coupled from the signal path toground. In this way, the circuit illustrated in FIG. 3 is realized. Itcan be seen that the device layout of FIG. 2 integrates the shunt andseries configuration illustrated and described with reference to FIGS.1A and 1B into a single device.

As a result, the dynamic range of the device 100 is enlarged withoutcompromising the switch size. The device layout also leads to a lowerswitching voltage combined with a faster switching time. The improvedperformance of the bi-stable device layout of FIG. 2 compared to a(conventional) single series or shunt switch (as described withreference to FIG. 1) is illustrated graphically in FIG. 4 of thedrawings. It can be seen that a bi-stable switch according to thepresent invention, as shown as an example only with reference to FIG. 2of the drawings, leads to a significantly higher isolation for a giveninsertion loss. For example, the series and shunt switches have anisolation of −20 dB for −0.1 dB of insertion loss. However, thebi-stable switch of FIG. 2 with an insertion loss of −0.1 dB has anisolation of −32 dB. It can also be seen that this enhanced performanceis achieved without increasing the overall device dimensions withreference to prior art devices.

The switch performance can be further optimized by changing the ratio ofthe bottom and top electrode areas. For example, in the event that aninsertion loss of −0.1 dB is allowed, the ratio of the bottom and topelectrode areas can be tuned to achieve the maximum isolation. Thisleads to a further increase in the isolation, in this case up to −38 dB,as illustrated graphically in FIG. 4 of the drawings. It should be notedthat these results were achieved by making C₁=2C₂, the data wascalculated for a frequency of 900 MHz (GSM band) with a gap between thetop and bottom electrode of 2.4 μm. It will be appreciated that theelectrode length is equal to the square root of electrode area if squareelectrodes are assumed.

Referring to FIG. 5 of the drawings, one way of altering the electrodearea is, for example, to segment the bottom electrode 20. However, othermethods of achieving the same result are envisaged and the invention isnot intended to be limited in this regard.

Because the switch of the present invention is bi-stable, theintermediate beam 220 can be designed to have a relatively lowstiffness, which is not the case for conventional switches because, inthat case, the movable electrode is required to be self-supporting whenthe switch is in the open state. Since, in the case of the presentinvention, the stiffness of the middle electrode can be low, this allowsfor very low actuation voltages (preferably below battery voltage of afew volts). Furthermore, the bi-stable switch of the present inventionleads to an increase in switching speed. For conventional switches, thespeed at which the movable electrode is released from the contactingelectrode depends only on the spring contact of the movable electrode.Therefore, if the switching speed of a conventional switch is requiredto be relatively fast, then the movable electrode must have a relativelyhigh stiffness, which in turn requires a high actuation voltage.However, in the case of the bi-stable switch of the present invention,the speed at which the middle electrode 220 can be detached from thecontacting electrode 20, 30 is increased by applying a voltage betweenthe middle electrode 220 and the opposing electrode 20, 30, such thatthere is no need for the movable electrode to have a high springconstant.

In general, in the process of manufacturing the electronic devicecomprising a MEMS element, firstly one or more sacrificial releaselayers 300 are provided and patterned adequately. Then a top layer 12 isprovided, by sputtering or PVD deposition of an aluminum layer at raisedtemperature (say 400 degrees centigrade). This top layer 12 isrelatively thick, compared with the normal thickness of thin films,generally 1-10 micrometers. This layer includes any interconnects andsupport structures to the substrate 14 (however, it will be appreciatedthat several different configurations are envisaged in this respect, andthe invention is not intended to be limited in this regard).

Next, the top layer 12 is structured using photolithography and etching,for example, wet chemical etching, so as to define first electrode 30.Then the release layer 300 is removed in, for example, a furtherchemical etching step, so as to render the first electrode 30 and thethird electrode 220 free-standing. Following this, the MEMS element 10is packaged hermetically, since moisture and the like tends to have adetrimental effect on the functionality of the device. Such packagingmay generally be achieved by means of a solder ring, and implemented byproviding the solder first, and thereafter reflowing the solder bypassing the device through a reflow oven at about 250-300 degreescentigrade.

Referring to FIG. 6 of the drawings, a method of manufacturing thedevice of FIG. 2 will now be briefly elaborated upon. The substrate 14is provided with a thermal oxide layer 142 on top of a high-ohmicsilicon substrate 141. Hereon a first layer of aluminum is provided,with a thickness of about 0.3 μm. This layer is structuredphotolithographically according to a desired pattern. This results inthe second electrode 20, which in this example is segmented. Hereon, afirst sacrificial layer 300 of silicon nitride is provided in athickness of again approximately 0.5 μm. On top of this firstsacrificial layer 300 a second metal layer is provided according to adesired pattern. The pattern includes the third electrode 220. Thereon,a further sacrificial layer 301 is applied, again of silicon nitride,which is deposited using PECVD. This layer 301 has a thickness of forinstance 1 μm. If desired, a further insulating layer 302 (shown in FIG.7) can be applied on top of this layer. This is particularly suitable ifthe MEMS element is integrated with a passive network and the body 12 isalso used for definition of other components. This further insulatinglayer 302 comprises a different material than the first and secondsacrificial layers 300, 301, for instance silicon oxide, an organicallymodified mesoporous silica, such as known from WO-A 03/024869,benzocyclobutene, or a photoresist material. This further insulatinglayer 302 will be patterned such that it is absent in the area of theMEMS element. Before or after this patterning, the second sacrificiallayer 301 will be patterned with reactive ion etching according to adesired pattern. The pattern creates windows to the first and secondmetal layer. Then a third metal layer 12 is provided, which will alsofill the windows. This metal layer of Al_(0.98)Cu_(0.02) of about1.0-1.5 μm is given a beam-like structure so as to define the firstelectrode 30. The first electrode 30 preferably has a bridgingstructure. Thereafter, a further photosensitive layer is applied. It ispatterned such that it creates windows to the selective locations of thefirst and second sacrificial layers 300, 301. The sacrificial layers300, 301 can thereafter be removed by using a plasma, in particular afluorine-based plasma. In this case, the insulating top layer 142 of thesubstrate further comprises an etch stop layer against such plasma, forinstance a layer of Al₂O₃ It is noted that it is alternatively possibleto use wet-chemical methods, or combinations of wet- and dry-chemicaletching.

As stated above, a dielectric layer 240, 180 covering the metal layersis used to avoid a short circuit between the electrodes when the middleelectrode 220 is pulled to one of the fixed electrodes 30,20. In thecase of aluminum (Al) being used to define the electrodes 30, 20, 220,the native Al-oxide functions as the dielectric (bearing in mind thatthe breakdown voltage of native Al-oxide>7V, as deduced fromexperimental data).

FIG. 7 shows a second embodiment of the device of the invention, whichcomprises a MEMS element 10, a thin film capacitor 50 and a verticalinterconnect 60. This figure illustrates the advantageous feature of theinvention, that the MEMS element having three electrodes 30,20, 220 canbe embedded in a passive network that comprises other components aswell, and without the need to apply any additional metal layer, orsacrificial layer. In fact, the first sacrificial layer 300 functionsalso as a dielectric of the thin-film capacitor 50. The electrodes 51,52of the thin-film capacitor 50 are defined in the same metal layers asthe second and the third electrode of the MEMS element 10. The thirdmetal layer 12 is not only first electrode 30, but also interconnect. Itis herein of particular importance, that the first and secondsacrificial layers 300, 301 are selectively etched away. It is therewithimproved in that not just one aperture in the body 12 is present, but aplurality of apertures; and in that the supporting structure has asubstantial extension, i.e. it is primarily wall-shaped and notpillar-shaped.

FIG. 8 shows a third embodiment of a MEMS element in a cross-sectionalview. FIG. 9 shows a top view of this embodiment, wherein the first,second and third electrodes 30, 20, 220 are shown on top of each other.Herein, the third electrode 220 is elastic, which means that itsmechanical force can be overcome by application of a driving voltage.The second electrode 20 is herein patterned so as to contain a pluralityof triangular segments. As a result, the surface area of the secondelectrode 20 is different from that of the first electrode 30. The firstand second electrodes 30,20 are furthermore provided with surface layers240, 180 of dielectric material, as the present example is a variablecapacitor. The position of the third electrode 220 is the result of thebalance of forces. These forces include primarily Van der Waals forces,electromagnetic forces and the internal mechanical force of the thirdelectrode 220. Since this electrode 220 is relatively thin and flexible,its internal mechanical force can be neglected to a large extent. Theposition can then be defined and changed by variation of the ratio ofthe voltages applied to the first and the second electrodes 30,20. Thesurface layers 180, 240 provided on the first and second electrodes30,20, and particularly the surface roughness thereof, will stronglydetermine the strength of the Van der Waals forces, and therewith theresistance against any change of position. Due to the triangularsegmentation of the second electrode 20, the nett electrostaticattraction force between the second and the third electrode 20, 220depends on the in-plane coordinates. In other words, this triangularsegmentation provides an embedded tendency that at the one end—left inthe drawing—of the third electrode 220 it will be attached to the first,top electrode 30. At the other end of the third electrode 220, the forceto the second electrode 20 is much larger, however, and the thirdelectrode 220 will be attached to the second electrode 20. Herewith, amechanism is provided which ensures that the third electrode 220 isnever completely sticted to either the second electrode 20 or the firstelectrode 30.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe capable of designing many alternative embodiments without departingfrom the scope of the invention as defined by the appended claims. Inthe claims, any reference signs placed in parentheses shall not beconstrued as limiting the claims. The word “comprising” and “comprises”,and the like, does not exclude the presence of elements or steps otherthan those listed in any claim or the specification as a whole. Thesingular reference of an element does not exclude the plural referenceof such elements and vice-versa. In a device claim enumerating severalmeans, several of these means may be embodied by one and the same itemof hardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

1. An electronic device comprising a microelectromechanical system(MEMS) element (10), the element comprising first (30) and second (20)electrodes and an intermediate beam (220) with first and second opposingconductive side faces, the first side face (260) facing the firstelectrode (30) and the second side face (280) facing the secondelectrode (20), which beam (220) is movable by application of a drivingvoltage between said first (30) and second (20) electrodes,characterized in that: the second electrode (20) and the secondconductive side face (280) of the beam (220) form with an intermediatedielectric a first switchable capacitor (C1) that is connected in asignal path between an input and an output, and the first electrode (30)and the first side face (260) of the beam (220) form with anintermediate dielectric a second switchable capacitor (C2), that iscoupled from the signal path to ground.
 2. An electronic device asclaimed in claim 1, wherein the beam (220) is embodied as a thirdelectrode.
 3. An electronic device as claimed in claim 1, wherein thefirst electrode (30) has a surface area that is larger than that of thesecond electrode (20).
 4. An electronic device as claimed in claim 3,wherein the second electrode (20) is subdivided into individualsegments.
 5. An electronic device as claimed in claim 1, wherein theelectrodes (20,30,220) are present in planes substantially parallel to asubstrate (14).
 6. An electronic device as claimed in claim 5, whereinthe second electrode (20) is present between the beam (220) and thesubstrate (14) and the first electrode (30) is embodied in a layer witha spring constant that is substantially larger than the spring constantof the beam (220).
 7. An electronic device as claimed in claim 1,wherein the conductive side faces (260, 280) of the beam (220) areconnected to the input and the first electrode (30) functions as theoutput.
 8. An electronic device as claimed in claim 2, wherein the thirdelectrode (220) is provided with an electrically insulating layer (240)at both the first and the second side faces (260, 280).
 9. An electronicdevice comprising a microelectromechanical systems (MEMS) element (10)provided on a substrate (14), comprising first (30) and second (20)electrodes, which electrodes (20,30) are provided in planes that aresubstantially parallel to the substrate (14), an intermediate beam (220)being provided between said first (30) and second (20) electrodes, saidintermediate beam (220) having first and second opposing conductive sidefaces (260, 280), the first side face (260) facing the first electrode(30) and the second side face (280) facing the second electrode (20),which beam (220) is movable by application of a driving voltage betweensaid first (30) and second (20) electrodes; characterized in that thefirst and second conductive side faces (260, 280) are part of the sameelectrically conductive layer being a third electrode (220).
 10. Anelectronic device as claimed in claim 9, wherein the second electrode(20) is present between the third electrode (220) and the substrate (14)and the first electrode (30) is embodied in a layer with a springconstant that is substantially larger than the spring constant of thethird electrode (220).
 11. An electronic device as claimed in claim 9,wherein the second electrode (20) is provided with a surface area thatis smaller than that of the first electrode (30).
 12. An electronicdevice as claimed in claim 11, wherein the second electrode (20) issubdivided into individual segments.
 13. An electronic device as claimedin claim 2, wherein the third electrode (220) is substantially elastic,such as to be attachable with a first surface area at one edge to thesecond electrode (20) and with a second surface area at an opposite edgeto the first electrode (30), and such that on application of anactuation voltage the ratio of first to second surface area ischangeable.
 14. An electronic device as claimed in claim 6, wherein thefirst electrode (30) is defined in a layer in which also an inductor isdefined.
 15. An electronic device as claimed in claim 2, wherein thefirst (30) and the third (220) electrodes are defined in layers, inwhich also the electrodes of a thin film capacitor are defined.
 16. Anelectronic device as claimed in claim 6, characterized in that the firstelectrode (30) is constructed as a bridge with supporting spacers on thesubstrate (14).
 17. An electronic device as claimed in claim 6, whereinthe first electrode (30) is part of a membrane- or bridge-likeconstruction that is supported on the substrate (14) with a number ofbeams laterally connected to said construction, therewith including aspring-like functionality that allows controlled displacement of thefirst electrode (30) in directions substantially perpendicular to thesubstrate (14).
 18. An electronic device as claimed in claim 1, whereinthe MEMS element (10) is part of an impedance matching network.
 19. Afront end module provided with a power amplifier and an electronicdevice (10) according to claim
 1. 20. Use of the electronic deviceaccording to claim 1, for RF applications, wherein the beam (220) isdriven by a driving voltage towards or from the first electrode (30).21. A method of driving an electronic device as claimed in claim 1 byapplication of an actuation voltage.